The transportation chief said that, nearly 24 hours after the crash, time is a “concern” for investigators and that the region in eastern Ukraine needs to be secure for an effective investigation.

“It’s obviously a matter of concern,” Foxx said, when asked about the need to move with urgency to investigate the site. “And that’s why last night, the White House asked all the parties involved to calm the situation down, de-escalate and allow the international community to come in and do an investigation. We stand ready to help.”

U.S. and foreign officials are calling for a full investigation into Flight 17 from Amsterdam to Kuala Lumpur that killed 298 people and that international officials believe was shot down. The Boeing 777 was grounded in the Donetsk region of eastern Ukraine, about 25 miles from the Russian border.

He also said that he anticipates that the international community will likely follow the FAA’s lead in prohibiting airlines from flying over eastern Ukrainian airspace. On Thursday evening, the FAA issued a notice that it had officially prohibited U.S. flight operations over much of that airspace, including the Simferopol and Dnepropetrovsk flight information regions. This notice was preceded by an April FAA prohibition on flights over the Simferopol region after the Russian annexation of Crimea.

Foxx did not directly address comments made by former NTSB Chairwoman Debbie Hersman, who said Thursday evening that she would be reluctant to send civilian investigators to the crash site. “I would be very concerned if I was anyone sending my people in there that they need to be safe,” she said.